Electron tube having an improved filamentary cathode and support therefor and method of making same

ABSTRACT

An electron tube is disclosed comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid and an anode. Cathode support means are disclosed comprising a generally cylindrical, metallic stem, a hollow, metallic support cylinder electrically connected to the cathode and coaxially situated about at least a portion of the stem in spaced relation therewith, and two dielectric annuli interposed between the stem and cylinder in abutment therewith.

United States Patent 1 Polese [54] ELECTRON TUBE HAVING AN IMPROVED FILAMENTARY CATHODE AND SUPPORT THEREFOR AND METHOD OF MAKING SAME [75] Inventor: James P. Polese, Menlo Park, Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Nov. 21, 1968 [21] Appl. No.: 777,747

[52] US. Cl. ..313/277, 313/253, 313/269, 313/292, 313/350 [51] Int. Cl ..H01j l/l8, H0lj1/90, HOlj 9/18 [58] Field of Search ..313/272, 275-278, 313/292, 252, 253, 250, 341, 350; 256/34, 47; 245/1 [56] References Cited UNITED STATES PATENTS 3,225,246 12/1965 Hollingworth et al ..3l3/270 June 5, 1973 3,227,912 l/l966 Ganswindt et al ..3l3/34l 3,327,156 6/1967 Gerlach ...3l3/250 3,449,616 6/1969 Sarrois ..3l3/34l Primary ExaminerJohn W. Huckert Assistant Examiner-William D. Larkins Attorney-Stanley Z. Cole [57] ABSTRACT An electron tube is disclosed comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid and an anode. Cathode support means are disclosed comprising a generally cylindrical, metallic stem, a hollow, metallic support cylinder electrically connected to the cathode and coaxially situated about at least a portion of the stem in spaced relation therewith, and two dielectric annuli interposed between the stem and cylinder in abutment therewith.

16 Claims, 14 Drawing Figures PATEN mm 5 mm SHEET 2 OF 2 INVENTOR. YJAMES P. POLESE ELECTRON TUBE HAVING AN IMPROVED FILAMENTARY CATHODE AND SUPPORT THEREFOR AND METHOD OF MAKING SAME A method of making the just described cathode support means, in which the stem and support cylinder have a coplanar step, is also disclosed comprising the steps of placing a dielectric annulus having a coefficient of thermal expansion less than the metallic stem and cylinder on the coplanar steps with the inner surface of the annulus in abutment with the cylindrical stem and the outer surface of the annulus in abutment with the hollow support cylinder, the stem and cylinder thereby occupying a first position with respect to the dielectric annulus. Two metallic rings are then placed upon the annulus with one ring in abutment with the cylindrical stern and the other ring in abutment with the hollow support cylinder. Mechanical force is then applied to the two metallic rings to push them firmly against the dielectric annulus. The electron tube is then heated causing the metallic stem and cylinder to expand and move with respect to the dielectric annulus and rings to a second position with respect thereto. The metallic ring in abutment with 'the stem is brazed thereto at the second position, and the metallic ring in abutment with the cylinder is brazed thereto at the second position. The electron tube is then cooled causing the stem and cylinder to return to their first position with the dielectric annulus compressed between the converging rings and steps and locked securely to the stern and cylinder. The mechanical force is then removed from the metallic rings.

An electron tube is also disclosed comprising a directly heated cathode having a plurality of filaments affixed to a cylindrical band having a plurality of flexible tabs. The flexible tabs are joined to a cylindrical surface of a cathode support member.

An electron tube is further disclosed having a directly heated cathode comprising two sets of parallel filaments wound into helices, the filaments of each set crossing the filaments of the other set to form a plurality of filament crossings. A minority of the crossings consists of two filaments in abutment whereas a majority of the crossings consists of two filaments fused together.

BACKGROUND OF THE INVENTlON conditions is quite substantial.

When the cathode filaments form a cylindrical mesh, deformation of the structure occurs when the cathode is subjected to the elevated temperatures encountered during carbonization or normal operation. This is caused by the fact that the structure rigidly supporting each end of the cathode heats and cools less rapidly than the small filaments, thereby causing-them to bulge or become over-tensional. This action creates stresses in the filaments and causes permanent deformation if the elastic limit of some of the filaments has been exceeded, or if some of the spot-welds have broken loose. The cylindrical mesh will often evidence such permanent deformation by assuming an hourglass shape upon cooling. This shape will, of course, alter the interelectrode spacing between the cathode and grid, and, in ex treme cases, may even create a cathode-to-grid short.

In addition, the filaments which are usually made of thoriated tungsten recrystallize and become brittle at the elevated temperatures encountered during tube fabrication and operation. Where filaments cross and are welded together to add rigidity to the mesh, this embrittlement is particularly pronounced. Furthermore, where the filaments acquire a permanent set at elevated temperature, they will retain this set at cooler temperatures when the mesh may have assumed an altered position due to contraction of the filaments. This distortion creates strains in the filaments, which already being brittle, may in turn cause individual welds or filaments themselves to pop or crack. This, of course, will adversely affect the cathode-to-grid spacing, and may even create a cathode-to-grid short circuit and cause filament failure.

The above problems may be alleviated by imparting axial flexibility to the cylindrical, mesh cathode to alleviate residual stresses and strains. One method of accomplishing this is to use finer filaments. This approach, however, is limited by both tube electrical parameters, and by the added difficulty of spot-welding at filament crossings and carburizing. Another manner. of accomplishing this is to use fewer filaments thereby increasing the size of the interstices bounded by the filaments. However, tube electronic specifications again limit this approach. Yet another way to add flexibility is the use of relatively low helix angles by the filaments. Again this adversely affects the electrical character of the cathode since the length and thus resistance of each filament is significantly increased. Furthermore, this procedure increases the number of filament crossings which must be spot-welded and creates strains in the wires since they must be bent more for a given diameter structure. Conversely, the use of a high helix angle adds rigidity rather than flexibility to the structure since the accordion action of the cylindrical mesh is reduced. In such configuration axial, compressive forces on the mesh cause the filaments to bulge; axial tension of the mesh is translated into filament tension. As a compromise the helical angle of the filaments in mesh cathodes should be confined within the range of approximately 20 to 30.

In addition to the filamentary cathode itself, the structure supporting the cathode within the electron tube also plays a major role in maintaining the interelectrode spacing of the cathode and control grid. The cathode support is more massive than the filaments themselves, and usually comprises materials having a different coefficient of thermal expansion. The rate of heating and cooling, and the stable operating temperature of the filaments and their supports, are at variance from each other. These diverse characteristics cause the cathode and its support to expand and contract at different rates which in turn may create adverse and non-uniform strains in the mesh cathode or cause it to bulge or constrict or otherwise alter their spacing with adjacent grid wires. Furthermore, mounting the cathode to its support during tube assembly may deform the fragile, filamentary structure.

Accordingly, it is the general object of the present invention to provide an electron tube having an improved filamentary cathode and supporting structure therefor.

A more specific object of the invention is the provision of a directly heated power grid electron tube having more uniform interelectrode spacing between the filamentary cathode and control grid.

Another object of the invention is to provide an electron tube having an improved mesh cathode.

A more particular object of the invention is to provide an electron tube having a cylindrical, mesh cathode having improved axial flexibility.

Yet another object of the present invention is the provision of an electron tube having improved support means for a directly heated cylindrical cathode, and a method of making same.

Otherobjects of the invention will become apparent upon a perusal of thedrawing and detailed description of the preferred embodiments.

SUMMARY OF THE INVENTION Briefly described, the present invention is an electron tube and its method of manufacture, the tube comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid and an anode, and cathode support means.

One feature of the invention is cathode support means comprising a generally cylindrical, metallic stem, and a hollow, metallic support cylinder electrically connected to the cathode and coaxially situated about at least a portion of the stem in spaced relation therewith. Two dielectric annuli are interposed between the stem and cylinder in abutment therewith.

Another feature of the present invention is a method of making an electron tube having the just described cathode support means in which the stem and cylinder have a coplanar step. This method comprises the steps of placing a dielectric annulus having a coefficient of thermal expansion less than the metallic stem and cylinder on the coplanar steps with the inner surface of the annulus in abutment with the cylindrical stem and the outer surface of the annulus in abutment with the hollow support cylinder, the stern and cylinder thereby occupying a first position with respect to the dielectric annulus. Two metallic rings are then placed upon the annulus with one ring in abutment with the cylindrical stem, and the other ring in abutment with the hollow support cylinder. Mechanical force is then applied to the two metallic rings to push them firmly against the dielectric annulus. The electron tube is heated causing the metallic stem and cylinder to expand and move with respect to the dielectric annulus and rings to a second position with respect thereto. The metallic ring in abutment with the stem is then brazed to the stem at the second position, and the metallic ring in abutment with the cylinder is brazed to the cylinder at the second position. The electron tube is then cooled causing the stem and. cylinder to return to their first position with the dielectric annulus compressed between the-converging rings and steps and thereby locked securely to the stem and support cylinder. The mechanical force is then removed from the metallic rings.

Another feature of the invention is an electron tube having a directly-heated cathode comprising a plurality of filaments affixed to a cylindrical band having a pluconsists of two filaments in abutment whereas a majority of the crossings consists of two filaments fused together.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view in elevation of an electron tube having an improved filamentary cathode and supporting structure therefor in accordance with the present invention.

FIG. 2 is a detailed, fragmentary, cross-sectional view in elevation of the cathode support structure of the electron tube shown in FIG. 1.

FIG. 3 is a cross-sectional view in elevation of a fragment of an alternative cathode support structure.

FIG. 4 is a plan view of a ceramic annulus comprising a portion of the structure shown in FIG. 2.

FIG. 5 is a fragmentary view in cross-section of another portion of the structure shown in FIG. 2.

FIGS. 6a and 6b are fragmentary views in crosssection of a portion of the structure shown in FIG. 1.

FIG; 7a is a fragmentary view in elevation of the cathode incorporated into the electron tube shown in FIG. 1. FIG. 7b is a fragmentary view in cross-section of the cathode support structure.

FIGS. 8a and 8b are greatly enlarged fragmentary views in elevation of a portion of the cathode illustrated in FIG. 7a.

FIG. 9a is an elevational view of a filamentary mesh cathode made without application of some principles of the present invention.

FIG. 9b is an elevational view of such a cathode made in accordance with the present invention.

FIG. 10 is an elevational view of a portion of the cathode structure shown in FIG. 9b with symbolic designations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in more detail to the drawing, there is illustrated in FIG. 1 an electron tube having four coaxially mounted, cylindrical electrodes consisting of a directly heated cathode 10, a control grid 11, a screen grid 12 and an external anode 13. The cathode and grids are mutually aligned and supported by means of supporting structure comprising a cylindrical, copper stem 16 to which center rod 17 is coaxially joined. The upper end of the cylindrical cathode is attached to this center rod by means of molybdenum cup 18. The lower end of the cathode is attached to a molybdenum cup 19 which is brazed to a hollow, copper support cylinder 20 which in turn is locked to stem 16 by means of two ceramic annuli 22 and 24. A flexible, copper collar 26 is affixed to the lower end of the support cylinder. The collar extends in sandwiched fashion between two of three ceramic rings 28 through the tube envelope to provide an externally accessible cathode terminal. Ce-

ramic rings 28, stem 16 and external anode 13 each form portions of the tube envelope as do ceramic cylinder 30 and metallic ring 32. An appropriate jacket, not shown, may be placed about the anode on mounted flange 34 for vapor or water cooling of the anode.

The lower cathode support structure of the tube illustrated in FIG. 1 is shown in greater detail in FIG. 2. In assembling this structure ceramic annulus 22 is seated upon stem step 41. Ceramic annulus 24 is seated upon stem step 42 which partially defines stem groove 43. As each annulus consists of three co-planar, spaced ceramic arcs, for reasons which will be hereinafter explained, no difficulty is encountered in their seating. Hollow support cylinder is then placed about the ceramic annuli with step 45 thereof supporting a portion of the bottom of ceramic annulus 22. As a result of this emplacement, steps 41 and 45 become co-planar.

Next two metallic rings 47 and 48 are placed in spaced relation upon ceramic annulus 22 with ring 48 in abutment with stem 16, and ring 47 in abutment with support cylinder 20. Following this two annular weights, which are illustrated by broken lines and designated W, are placed upon rings 47 and 48 respectively. The force provided by the weight of weights W push rings 47 and 48 firmly against ceramic annulus 22. The assembly is then heated. Support cylinder 20 and stem 16 being copper expand more than ceramic annulus 22 which has a lower coefficient of thermal expansion. As a result portions of the cylinder and stem slide upwards along the annular surfaces of annulus 22, and assume a new position with respect thereto. At this point metallic rings 47 and 48 are brazed to support cylinder 20 and stem 16, respectively. The assembly is then cooled causing the support cylinder and stems to contract. Metallic rings 47 and'48, being now affixed to the cylinder and stem, are forced downwardly towards steps 41 and 45. Ceramic annulus 22, being interposed between the converging rings and steps, is securely locked therebetween in compression. Weights W are then removed.

The resulting structure is an improvement over groove-type locks such as that shown in FIG. 3. Here there must exist a very slight vertical clearance between the annulus and support cylinder for assembly which clearance increases at elevated tube operating temperatures due to the fact that the copper stem expands more than the ceramic annulus. This of course makes the cathode support less secure and susceptible to axial vibration and shock. In addition, the machining required to achieve the close tolerance involved is quite expensive.

Another feature of the support assembly is that of the sliding capability of support cylinder 20 during thermal cycling. During heating the lower portion of the cylinder will expand downwardly in addition, of course, to radially. As the cylinder is in sliding contact with lower annulus 24 rather than being affixed thereto, the portion in contact slides downwardly along the outer surface of the annulus. Yielding to this movement, collar 26 may flex downwardly.

The reason for the segmentation of ceramic annuli 22 and 24 can be appreciated by reference to FIG. 4. Here annulus 22 is shown to consist of three circularly disposed, ceramic arcs 23. Were annulus 22 to be a single, integral ring, its thermal expansion in the plane of the drawing would be a function of its diameter Y. However, by segmenting the annulus into three distinct arcs, such expansion becomes a function of material thickness x. As x is substantially smaller than y, the expansion of the annulus is accordingly reduced. Were the annuli not segmented, gaps would have to exist between the inner surfaces thereof and the stem to allow for stem expansion during thermal cycling. Furthermore, at elevated temperatures gaps would exist beT tween the outer arcuate surfaces of the annuli and the support cylinder.

FIG. 5 illustrates stem 16 joined to support cylinder 20 by means of but one ceramic annulus 22, and

therein the justification for the use of a second ceramic annulus. Without lower annulus 24, support cylinder 20 is free to rock back and forth as indicated by the arrows. Such motion causes the lower portion of the cathode 10 to move with respect to the tube control grid, and adversely affect their spacing. For the sake of clarity this motion has been shown exaggerated. The inclusion of lower annulus 24 serves to prevent this rocking motion while nevertheless permitting axial movement by the support cylinder as above explained.

FIGS. 6a and 6b show in greater detail the attachment of support cylinder 20 to cup 19. The cylinder is preferably made of copper and the cup of molybdenum. The support cylinder therefore has a larger coefficient of thermal expansion. As shown in FIG. 6a cylinder 20 occupies position e initially in the process of affixing it to cup 19. When the structures are heated the cylinder expands till it abuts and is brazed to the cup as shown by position f. The structures are then cooled. Cylinder 20 then contracts till it occupies final position g. Were the cylinders to be placed in abutment initially, upon heating support cylinder 20 would expand more than up to 19 and assume the position illustrated in FIG. 6b. Note the existence of a gap between the upper portion of the support cylinder and the cup. The presence of this gap would prevent the making of an effective braze between the two members at elevated temperatures. Slits are provided to add flexibility to the support cylinder during thermal cycling.

FIGS. 7a and 7b show in greater detail the means and manner by which the filamentary cathode itself is attached to upper cup 18, which is duplicated in its attachment to lower cup 19. Cathode 10 is shown to consist of a plurality of thoriated tungsten filaments the ends of which are welded to two thin, molybdenum bands 52 having a plurality of flexible, integral tabs 53. The tabs are individually welded to cups l8 and 19 by means of spot-welder 54 and a half-mil platinum ribbon inserted between the cup and band. Were tabs 53 not present, the joinder of the bands to the-cups would not be as circumferentially uniform since the mechanical force applied by the spot welder would be translated from that spot on the band toother areas thereby creating residual stresses which could distort the band rendering it nonconcentric. By forcing the spot. welder against individual tabs, however, as shown in FIG. 7b, the tabs themselves flex. As a result filaments 55 will more truly define a cylindrical structure after assembly to cups l8 and 19 thereby providing more precise interelectrode spacing with the adjacent control grid.

FIGS. 8a and 8b illustrate the manner by which individual filaments 55 are affixed to cathode band 52 to provide improved cathode-to-grid spacing. Two individual, overlapping filaments 55 are shown in each figure welded to band 52 at weld points w. In FIG. 8a a force F has been applied to the overlapping filament causing that filament to be crimped. This crimping has not been done to the overlapping filament in FIG. 8b

however, with the results that it projects from the band at an angle 6. The existence of this angle causes the cathode to assume a slight conical shape in its vicinity adjacent bands 52 which, of course, is a departure from the desired cylindrical shape. Furthermore, this contributes to the formation of an hourglass shape of the cathode. By the applications of Force F, as shown in FIG. 8a, the tendency towards this formation has been substantially reduced.

The effect of the aforementioned hourglass shape of prior art mesh cathodes can be more fully appreciated by reference to FIGS. 9a and 9b. In FIG. 9a cathode 10 exhibits an hourglass shape which shape has been somewhat exaggerated for purposes of illustrations. Adjacent control grid 11, however, exhibits a true cy lindrical shape. As a consequence the spacing between these two electrodes varies. Cathode 10 in FIG. 9b, on the other hand, is more in the shape of a true rightcircular cylinder. The spacing between it and control grid 11 is therefore more uniform. This permits both closer spacing between the cathode and control grid, and provides sound tube electronic performance. It is the cathode shape seen in FIG. 9b which may be obtained through application of the principles of the present invention.

FIG. 10 shows a portion of cylindrical cathode 10 in greater detail as comprising two sets 58 and 59 of parallel filaments wound into helices about a common axis. The filaments of each set are seen to cross at least some of the filaments of the other set. The filaments have been welded together at a majority of these crossings as symbolically indicated by the black dots. A minority has not been welded as indicated by their absence of dots. Those not fused together merely lie in abutment with one another.

Both the fused and non-fused crossings define parallel rows of crossings such as those axially consecutive three designated R1, R2 and R3 in the drawings. In a complete, cylindrical cathode the crossings will, of course, actually define parallel circles. Row R1 consists of non-fused crossings while rows R2 and R3 are seen to consist of fused crossings. This sequence is repeated throughout the axial length of the cathode. In other words, every third row consists of non-fused crossings while the balance consist of fused crossings. This novel structure provides a mesh cathode which will maintain its cylindrical shape during thermal cycling as it has axial flexibility. This flexibility aids in preventing the filaments from taking a permanent set during carbonization or tube operation which would produce residual strains at lower temperatures upon filament contraction thereby placing the filaments in jeopardy of fracturing or having their spot-welds pop loose. Furthermore, the effort required in spot-welding the numerous number of crossings is, of course, reduced by a third.

At tube operating temperatures the cathode will acquire the shape of the cathode shown in FIG. 9b and consequently have uniform, predetermined interelectrode spacing with an adjacent, control grid. 0n the other hand, were the crossings of every other row to be welded the cathode would comprise, in effect, two independent, cylindrical meshes which could vibrate against each other and thereby create tube noise. The same would hold true with other even multiples of fused and non-fused rows. With odd intervals however this is avoided. With every third row consisting of nonfused crossings a sound compromise between axial flexiblity and radial rigidity is reached. It should however be appreciated that other odd-interval combinations are possible such as every third row consisting of fused crossings, or every fifth row consisting of non-fused crossings, and so forth.

It should also be understood that the above-described embodiments are merely illustrative of applications of the principles of the invention. Obviously, many modifications may be made to the illustrated examples without departing from the spirit and scope of the invention as set forth in the following claims:

What is claimed is:

1. An electron tube comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid and an anode, and cathode support means comprising a generally rotationally symmetrical conductive body, said cylindrical cathode having one end thereof joined to one end of said support means concentric therewith, said support means including at the other end thereof a generally cylindrical metallic stem, a hollow, metallic support cylinder electrically connected to the other end of said cathode and coaxially situated about at least a portion of said stem in spaced relation therewith, and two dielectric annuli interposed in axially spaced relationship between the cylindrical surface of said stem and the inner cylindrical surface of said support cylinder in abutment with each of said surfaces, the one of said annuli proximal said other end of said cathode being axially fixed to both said stem and said support cylinder, the other of said annuli being axially free with respect toat least one of said support cylinder and stem members.

2. An electron tube in accordance with claim 1 wherein said stem has an annular groove into which one of said annuli is seated.

3. An electron tube in accordance with claim 1 wherein said stem and cylinder have a coplanar step upon which one of said annuli is supported.

4. An electron tube in accordance with claim 1 wherein said cathode support means further comprises a flexible, metallic collar'fixed about one end of said cylinder and to said envelope.

5. An electron tube in accordance with claim 4 wherein said collar extends through said envelope to the exterior thereof where it provides a terminal for said cathode.

6. An electron tube in accordance with claim I wherein one end of said cylinder adjacent said cathode has a plurality of spaced slits.

7. An electron tube in accordance with claim 6 wherein said cathode support further comprises a cylindrical rim fixed about the outer periphery of the slitted end of said cylinder.

8. An electron tube in accordance with claim 1 wherein said cathode support further comprises a first metallic ring in abutment with an end of one of said annuli and brazed to the inner surface of said cylinder, and a second metallic ring in abutment with said end of said one annulus and brazed to said stem.

9. An electron tube in accordance with claim 1 wherein each of said annuli consist of a plurality of coplanar, spaced arcs.

10. An electron tube comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid, and an anode, and cathode support means, said directly heated cathode having a plurality of filaments affixed to a cylindrical band having a plurality of flexible tabs, said cathode support means having a member having a cylindrical surface to which said flexible tabs are joined.

11. An electron tube in accordance with claim 10 wherein said cathode support means comprises a generally cylindrical metallic stem, a hollow, metallic support cylinder electrically connected to said cathode and coaxially situated about at least a portion of said stem in spaced relation therewith, and two dielectric annuli interposed between said stern and cylinder in abutment therewith,

12. An electron tube comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid and an anode, and cathode support means, said directly heated cathode comprising two sets of parallel filaments wound into helices, the filaments of each set crossing at least some of the filaments of the other set to form a plurality of filamentcrossings, said crossings being selectively fused in a pattern such that the fused crossings are grouped in rows comprising 11 fused crossings separated from adjacent rows of n fused crossings by at least one unfused crossing, wherein n is an even integer.

13. An electron tube -in accordance with claim 12 wherein n is 2.

14. An electron tube in accordance with claim 12 wherein said filaments are affixed to a cylindrical band having a plurality of flexible tabs, and wherein said cathode support means comprises a cathode support member having a cylindrical surface to which said flexible tabs are joined.

15. An electron tube in accordance with claim 12 wherein said cathode support means comprises a generally cylindrical metallic stem, a hollow, metallic support cylinder electrically connected to said cathode and'coaxially situated about at least a portion of said stem in spaced relation therewith, and two dielectric annuli interposed between said stern and cylinder in abutment therewith.

16. An electron tube in accordance with claim 15 wherein said cathode has a plurality of filaments affixed to a cylindrical band having a plurality of flexible tabs, and wherein said cathode support means comprises a cathode support member having a cylindrical surface to which said flexible tabs are joined. 

1. An electron tube comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid and an anode, and cathode support means comprising a generally rotationally symmetrical conductive body, said cylindrical cathode having one end thereof joined to one end of said support means concentric therewith, said support means including at the other end thereof a generally cylindrical metallic stem, a hollow, metallic support cylinder electrically connected to the other end of said cathode and coaxially situated about at least a portion of said stem in spaced relation therewith, and two dielectric annuli interposed in axially spaced relationship between the cylindrical surface of said stem and the inner cylindrical surface of said support cylinder in abutment with each of said surfaces, the one of said annuli proximal said other end of said cathode being axially fixed to both said stem and said support cylinder, the other of said annuli being axially free with respect to at least one of said support cylinder and stem members.
 2. An electron tube in accordance with claim 1 wherein said stem has an annular groove into which one of said annuli is seated.
 3. An electron tube in accordance with claim 1 wherein said stem and cylinder have a coplanar step upon which one of said annuli is supported.
 4. An electron tube in accordance with claim 1 wherein said cathode support means further comprises a flexible, metallic collar fixed about one end of said cylinder and to said envelope.
 5. An electron tube in accordance with claim 4 wherein said collar extends through said envelope to the exterior thereof where it provides a terminal for said cathode.
 6. An electron tube in accordance with claim 1 wherein one end of said cylinder adjacent said cathode has a plurality of spaced slits.
 7. An electron tube in accordance with claim 6 wherein said cathode support further comprises a cylindrical rim fixed about the outer periphery of the slitted end of said cylinder.
 8. An electron tube in accordance with claim 1 wherein said cathode support further comprises a first metallic ring in abutment with an end of one of said annuli and brazed to the inner surface of said cylinder, and a second metallic ring in abutment with said end of said one annulus and brazed to said stem.
 9. An electron tube in accordance with claim 1 wherein each of said annuli consist of a plurality of coplanar, spaced arcs.
 10. An electron tube comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid, and an anode, and cathode support means, said directly heated cathode having a plurality of filaments affixed to a cylindrical band having a plurality of flexible tabs, said cathode support means having a member having a cylindrical surface to which said flexible tabs are joined.
 11. An electron tube in accordance with claim 10 wherein said cathode support means comprises a generally cylindrical metallic stem, a hollow, metallic support cylinder electrically connected to said cathode and coaxially situated about at least a portion of said stem in spaced relation therewith, and two dielectric annuli interposed between said stem and cylinder in abutment therewith,
 12. An electron tube comprising an evacuated envelope enclosing at least three cylindrical electrodes including a directly heated cathode, a grid and an anode, and cathode support means, said directly heated cathode comprising two sets of parallel filaments wound into helices, the filaments of each set crossing at least some of the filaments of the other set to form a plurality of filament crossings, said crossings being selectively fused in a pattern such that the fused crossings are grouped in rows comprising n fused crossings separated from adjacent rows of n fused crossings by at least one unfused crossing, wherein n is an even integer.
 13. An electron tube in accordance with claim 12 wherein n is
 2. 14. An electron tube in accordance with claim 12 wherein said filaments are affixed to a cylindrical band having a plurality of flexible tabs, and wherein said cathode support means comprises a cathode support member having a cylindrical surface to which said flexible tabs are joined.
 15. An electron tube in accordance with claim 12 wherein said cathode support means comprises a generally cylindrical metallic stem, a hollow, metallic support cylinder electrically connected to said cathode and coaxially situated about at least a portion of said stem in spaced relation therewith, and two dielectric annuli interposed between said stem and cylinder in abutment therewith.
 16. An electron tube in accordance with claim 15 wherein said cathode has a plurality of filaments affixed to a cylindrical band having a plurality of flexible tabs, and wherein said cathode support means comprises a cathode support member having a cylindrical surface to which said flexible tabs are joined. 